1. Field of the Invention
The present invention relates to a novel steel for cold forging having both good formability in cold forging and good machinability. The invention encompasses a preferable method of making the steel for cold forging.
This invention is applicable to various steels such as machine structural carbon steel, manganese steel, chromium steel, molybdenum steel, chromium-molybdenum steel, nickel-chromium steel, nickel-chromium-molybdenum steel, manganese-chromium steel, and nickel-molybdenum steel.
2. State of the Art
Sulfur has been known as an element which deteriorates formability in cold forging of steels. Sulfur in the steel exists as sulfides such as MnS, which are easily extended along the forging direction to become strand-form, and the elongated sulfides are believed to be harmful to the formability in cold forging. Accordingly, conventional production of a steel for cold forging includes a step of desulfurization to form a low-sulfur steel. Low-sulfur steels, however, have relatively low machinability.
We have sought ways of solving the above problem and studied compositions of the steel for cold forging. As a result, we found that, when Te is added to a sulfur-containing steel in an amount where the ratio %Te/%S is at least 0.04, the elongation of the sulfides is remarkably suppressed, and therefore, that the steel has decreased anisotropy in mechanical properties, fairly good formability in cold forging and excellent machinability which is equivalent to, or even better than that of conventional sulfur-containing free cutting steels. Our discovery was disclosed in the U.S. patent application Ser. No. 77,477.
However, there has been further demand for a steel having more highly improved formability in cold forging with sufficient machinability.